ESR1 and breast cancer: Classified based on the expression of hormone (estrogen or progesterone) receptors (HR) and human epidermal growth factor receptor 2 (HER2), breast cancer includes four subtypes: (1) HR+/Her2− (luminal A); (2) HR+/Her2+; (3) HR−/Her2+; and (4) triple-negative breast cancer (TNBC), which is negative for estrogen receptor (ER), progesterone receptor (PR) and Her2 [21].